A hybrid vehicle has been developed to reduce emission and improve fuel economy. Japanese Patent No. 3305974 (U.S. Pat. No. 5,934,089) shows an air conditioning system for a hybrid vehicle. When a temperature of an air cooled by an evaporator is less than or equal to a specified value with the vehicle stopped, an internal combustion engine is not started. When the temperature of an air cooled by an evaporator is greater than the specified value, the internal combustion engine is started to drive a compressor of the air conditioning system.
Generally, in a control system of a hybrid vehicle, an engine power is determined based on a vehicle-driving power which the hybrid vehicle needs. A surplus power is used for driving an air conditioning system. However, in such a control system of a hybrid vehicle, when a vehicle-driving load is not high, it is likely that a thermal energy (heat quantity of engine coolant) for a heater system may run short.
Also, in another control system of a hybrid vehicle, an internal combustion engine generates energy for both of vehicle-driving and air-conditioning. However, in such a control system of a hybrid vehicle, when a vehicle-driving load is high, it is likely that an operating point of the internal combustion engine is brought into an outside of an area where the engine is operated efficiently. As a result, fuel economy may be deteriorated.
In a hybrid vehicle, since the internal combustion engine is intermittently driven, a thermal energy of the engine coolant easily runs short for a heating. Thus, it is necessary to generate the thermal energy only for a heating. Alternatively, it is necessary to generate an electric power for turning on an electric heater.
Even in the air conditioning system for a hybrid vehicle shown in Japanese Patent No. 3305974 (U.S. Pat. No. 5,934,089), the above described matters cannot be avoided.